In general, because it's a flag that says to do things in an incorrect but faster way. It's like -ffast-math. The applications for which it's intended don't do anything where the incorrectness matters. Some random application falsely labeled hl2.exe may or may not.

> What it should do is ensure some things not relevant to Half-Life 2 were not done, thus getting better performance for this game in particular, but there is no guarantee that same optimizations work for other applications or games, so one should not expect an overall improvement.

I can't quite parse this. Yes, there is no guarantee that the optimizations will work for another game, which is precisely why you can expect an improvement with hl2. With non-hl2, you may get an improvement, you may not, and you may get incorrect behavior.

Everything else is not the same, but hl2 doesn't use the stuff that's different.

I wouldn't be surprised if it made other games on the Source engine faster, but everything else slower.